This invention relates to golf balls and methods of manufacturing golf balls. The physical properties, composition, and design of a golf ball determine its performance characteristics during play, including distance, loft, flight symmetry, spin, and durability. Golf balls can be constructed so that certain performance characteristics are emphasized and/or deemphasized to execute various types of shots in view of the different golf clubs used to make such shots.
For example, golf balls can be constructed to emphasize or deemphasize the spin characteristic of the ball according to the distance of the desired shot and the club used to make such shot. For long distance shots, where drivers, woods, and long irons are used, high spin is generally not desirable. High spin can increase lift which can result in a loss of distance. In contrast, for mid to short distance shots, high spin (approximately 5,000 to 12,000 rpm) can be desirable. High spin can improve control during flight and stopping power (“grab”) when the ball hits the green. The characteristic of a ball to have low spin for long distance shots and high spin for mid to short distance shots can be referred to as spin separation. Multilayer golf balls with soft covers (low flex modulus and low hardness) and hard intermediate layers provide good spin separation.
However, soft covers are susceptible to “scuffing” or shear yielding of the polymer that compromises the integrity and performance of the ball. The irons and wedges used to perform mid to short distance shots have a clubface with a large angle of loft when measured in reference to the ground with 0 degrees being with the face of the club perpendicular to the ground and 90 degrees being with the face of the club parallel to the ground. The large angled clubfaces impart relatively high shear force on the cover of the ball when the clubface strikes the ball. These shear forces are high enough to cause the polymer material of the cover to shear yield or “scuff.” A high degree of scuffing renders the ball unfit for play. It can also cause the spin rate to decrease during the shot when the scuffing occurs, thus giving inconsistent spin characteristics to the golf ball. Accordingly, it is desirable to construct a golf ball cover that is soft for use in multilayer golf balls with good spin separation but less susceptible to shear yielding or scuffing.
One attempted solution is described in U.S. Pat. Nos. 6,458,307 and 6,729,975. These patents disclose hardening the surface of a golf ball cover made of thermoplastic resin by allowing a polyisocyanate compound to permeate into the surface of the cover and cause a reaction between the polyisocyanate compound and active hydrogen groups contained in the thermoplastic resin to form a number of urethane linkages and urea linkages to crosslink the thermoplastic resin. Nonetheless, scuffing continues to be problem.